Air pollutants and air quality standards

An adult breathes about 15,000 liters of air a day, far more by mass than the food and water they consume, and cannot choose to stop. Air pollution is one of the largest environmental causes of illness and early death worldwide. Health Canada estimates that air pollution above natural background contributes to about 15,000 premature deaths a year in Canada, and US studies attribute tens of thousands of deaths a year to fine particles alone. This lesson describes the main air pollutants, where they come from and how they affect health, how concentrations in air are expressed and converted, the ambient air quality standards in Canada and the United States, how emissions are estimated and inventoried, and the main ways of controlling them.

Willowbend's air job is a permit application for a new hot-mix asphalt plant on the edge of the city. The plant will dry and heat crushed stone in a rotary drum, mix it with liquid asphalt, and run mainly from May to October. Nadia must estimate its emissions, compare predicted concentrations with the standards (lesson 10) and describe the controls.

The main air pollutants

Governments regulate a small group of common pollutants with ambient standards. In the United States they are called criteria pollutants because the EPA must publish criteria documents summarizing the science behind each standard; Canada regulates a similar list through the Canadian Ambient Air Quality Standards.

Primary pollutants are emitted directly, such as SO₂ from a smelter stack or CO from a tailpipe. Secondary pollutants form in the atmosphere from precursors: ozone forms when sunlight drives reactions between nitrogen oxides and VOCs, and a large share of PM₂.₅ forms when SO₂, NOx and ammonia react to make sulfate and nitrate particles. This is why controlling ozone and fine particles means controlling emissions of several different pollutants, often far upwind.

Fine particles, those smaller than 2.5 micrometers, are the most harmful common pollutant because they penetrate deep into the lungs and some reach the bloodstream. Research has found no clear threshold below which they cause no harm. Wildfire smoke has become a major source in recent years; the record 2023 Canadian wildfire season sent smoke across much of North America and pushed PM₂.₅ in cities from Edmonton to New York to many times normal levels.

Beyond the common pollutants, hundreds of hazardous air pollutants, or air toxics, such as benzene, formaldehyde, mercury and dioxins, are regulated mainly through emission limits on the industries that release them. Greenhouse gases, chiefly carbon dioxide, methane and nitrous oxide, are a different kind of air problem: they are not directly toxic at ambient levels but warm the climate, and they are managed through carbon pricing, emission caps and reporting programs.

Expressing concentrations in air

Concentrations of particles are given as mass per volume of air, in micrograms per cubic meter (µg/m³). Gas concentrations are given either the same way or as a volume fraction, in parts per million (ppm) or parts per billion (ppb). Because equal volumes of gases at the same temperature and pressure contain equal numbers of molecules, a volume fraction is also a mole fraction. Converting between the two uses the ideal gas law. One mole of any gas occupies 24.45 L at 25 °C and 1 atm, so:

Here M is the molar mass in g/mol. At other temperatures and pressures, the molar volume changes in proportion to absolute temperature and inversely with pressure: at 0 °C it is 22.41 L, and at −20 °C, a typical prairie winter day, it is about 20.8 L. Standards usually specify the reference conditions, most often 25 °C and 101.325 kPa.

# Convert gas concentrations between ppb/ppm and ug/m3
def to_ugm3(ppb, M, T_C=25.0, P_kPa=101.325):
    Vm = 22.414 * (T_C + 273.15) / 273.15 * 101.325 / P_kPa   # molar volume, L/mol
    return ppb * M / Vm

gases = [("O3", 48.00, 62), ("NO2", 46.01, 100), ("SO2", 64.07, 75), ("CO", 28.01, 9000)]
for name, M, ppb in gases:
    print(f"{name:4s} {ppb:6.0f} ppb = {to_ugm3(ppb, M):8.0f} ug/m3 at 25 C"
          f"  ({to_ugm3(ppb, M, T_C=-20):8.0f} ug/m3 at -20 C)")

O3       62 ppb =      122 ug/m3 at 25 C  (     143 ug/m3 at -20 C)
NO2     100 ppb =      188 ug/m3 at 25 C  (     221 ug/m3 at -20 C)
SO2      75 ppb =      196 ug/m3 at 25 C  (     231 ug/m3 at -20 C)
CO     9000 ppb =    10304 ug/m3 at 25 C  (   12136 ug/m3 at -20 C)

Ambient air quality standards

Under the Clean Air Act, the EPA sets National Ambient Air Quality Standards (NAAQS): primary standards to protect public health, including sensitive groups such as children and people with asthma, with an adequate margin of safety, and secondary standards to protect public welfare, such as crops, visibility and buildings. The law requires the EPA to review each standard every five years. Areas that do not meet a standard are designated nonattainment, and the state must prepare a state implementation plan showing how it will come into attainment. In February 2024 the EPA lowered the annual primary PM₂.₅ standard from 12.0 to 9.0 µg/m³; the agency later asked the court to vacate its own rule, but in 2026 the D.C. Circuit upheld the standard and denied the request.

For the public, the EPA and state agencies report the Air Quality Index (AQI), which converts each pollutant's concentration into a score from 0 to 500 with color-coded categories from good (green) to hazardous (maroon); the reported AQI is the highest of the individual pollutant scores. South Dakota's air program applies the NAAQS and state rules when it reviews the asphalt plant's permit.

Monitoring air quality

Ambient concentrations are measured at fixed monitoring stations operated mainly by state and local agencies under EPA requirements, using federal reference or equivalent methods. The data feed the EPA's Air Quality System database, which is used to decide whether areas meet the NAAQS, and the AirNow system, which reports the AQI to the public in near real time. Low-cost sensors are now widely used by communities to map neighborhood differences, although their readings need correction against reference instruments, especially in humid or smoky conditions.

Indoor air deserves attention too. People in Canada and the United States spend about 90 percent of their time indoors, where radon, combustion gases from gas stoves and furnaces, wood smoke, mold and chemicals from furnishings and cleaning products can reach higher concentrations than outdoors. Ventilation, source control and filtration are the indoor equivalents of the controls used for outdoor air.

Emission inventories and emission factors

An emission inventory is a mass balance on an airshed: a list of how much of each pollutant every source releases in a year. Inventories cover point sources such as power plants and factories, area sources such as home heating and farms, mobile sources such as cars, trucks and trains, and natural sources such as wildfires. Large facilities measure some emissions with continuous emission monitoring systems in their stacks. For most sources, emissions are estimated with emission factors, average emission rates per unit of activity, such as kilograms of particulate per metric ton of asphalt produced or grams of NOx per kilometer driven. In the United States, the EPA's AP-42 compilation is the main source of emission factors; Canada's National Pollutant Release Inventory requires facilities above reporting thresholds to report their releases each year and publishes the data.

Here E is the emission rate, A the activity rate, EF the uncontrolled emission factor and ER the efficiency of the control device in percent.

Controlling air pollution

Control starts with prevention: cleaner fuels, efficient combustion, process changes and enclosures that stop emissions forming in the first place. After that, add-on devices treat the exhaust gas. Particles are removed by cyclones, which use centrifugal force and suit coarse dust; fabric filters or baghouses, which trap particles on cloth bags and reach very high efficiencies; electrostatic precipitators, which charge particles and collect them on plates; and wet scrubbers. Sulfur dioxide is removed by flue gas desulphurization, in which a lime or limestone slurry absorbs the SO₂. Nitrogen oxides are reduced by low-NOx burners and by selective catalytic reduction, in which ammonia reacts with NOx over a catalyst to form nitrogen and water. VOCs are destroyed by thermal or catalytic oxidizers or recovered by carbon adsorption. For vehicles, catalytic converters, cleaner fuels and tighter tailpipe standards have cut emissions per kilometer by well over 90 percent since the 1970s. Fugitive dust from stockpiles and haul roads at a site like the asphalt plant is controlled with paving, water sprays and wind fences.

The air quality gains in Canada and the United States since 1970 are one of the great successes of environmental engineering. Lead in air fell by more than 90 percent after leaded gasoline was phased out, sulfur dioxide fell sharply after the acid rain programs of the 1980s and 1990s and the 1991 Canada-United States Air Quality Agreement, and urban smog episodes became far less frequent, even as populations and the economy grew.